The gases in air, such as especially oxygen and nitrogen, are of great industrial importance, especially in the fields of papermaking or glassmaking.
One of the non-cryogenic techniques used for producing these gases is the technique called "PSA" (standing for Pressure Swing Adsorption), which technique covers not only the strictly speaking PSA processes, but also the similar processes such as the VSA (Vacuum Swing Adsorption) and the MPSA (Mixed Pressure Swing Adsorption) processes.
According to this PSA technique, when the gas mixture to be separated is air and the component to be recovered is oxygen, the oxygen is separated from the gas mixture by means of preferential adsorption of at least the nitrogen on a material preferentially adsorbing at least nitrogen and subjected to given pressure cycles in the separation zone.
The oxygen, not being adsorbed or only slightly so, is recovered as output from the separation zone; in general, this has a purity greater than 90%, or even greater than 93%.
More generally, a PSA process for the non-cryogenic separation of a gas mixture comprising a first component that is preferentially adsorbed on an adsorbent material and a second component less preferentially adsorbed on the adsorbent material than the first component, for the purpose of producing the second component, comprises, in a cyclic manner:
a step of preferential adsorption of at least the first component on the adsorbent material at an adsorption pressure called "high pressure", with recovery of at least some of the second component thus produced; PA1 a step of desorption of the first component, thus trapped by the adsorbent, at a desorption pressure below the adsorption pressure, called "low pressure"; PA1 a step of recompression of the separation zone comprising the adsorbent, by going from the low pressure to the high pressure. PA1 DP.sub.0 is the difference between the extreme pressures in nominal operation; PA1 T.sub.c is the cycle time or duration (in seconds) PA1 d is the time (in seconds) during which there is no production; and PA1 Y is the duration (in seconds) of the reduced-operation dead time. PA1 at least one feed step of feed duration (TA), which comprises at least one introduction of the gas stream to be separated into at least one adsorber; PA1 at least one production step, which comprises at least one recovery of a stream of gas produced at a prefixed production pressure (PP) and with a prefixed nominal production output (DN); and PA1 at least one pumping step of pumping duration (TP), which comprises at least one extraction of a gas stream from at least one adsorber; PA1 on the one hand, a setting of the nominal durations TA.sub.0 and TP.sub.0 so as to obtain a set production output/pressure/purity combination; and PA1 on the other hand, a control of the TA/TP pair so as to maintain, particularly under the best economic conditions, the purity/pressure pair for any output of gas consumed or actual output (DR) less than or equal to a nominal gas output (DN). PA1 TA: the feed duration, that is to say the time during which the gas mixture to be treated is brought into contact with a bed of adsorbent; PA1 TB: the pumping duration, that is to say the time during which the residual gas is removed from the adsorbent bed; PA1 DN: the nominal output of the unit; PA1 PP: the minimum pressure of the production gas; PA1 PG: the purity of the production gas; PA1 the indices 0 indicate the values taken by these variables under the initial setting conditions of the PSA unit. PA1 the effective feed duration (TA) is controlled as a function of the actual production pressure (PP) and the ratio (TP/TA) of the effective pumping duration to the effective feed duration is controlled as a function of the ratio (DR/DN) of the actual production output to the nominal production output; PA1 the effective pumping duration (TP) is furthermore controlled so as to maintain a set purity (PG) of the gas produced; PA1 during the feed step of initial feed duration (TA.sub.0), at least a first dead time of a first dead time duration (X) is introduced such that the actual feed duration (TA) is equal to TA.sub.0 -X; PA1 during the pumping step of initial pumping duration (TP.sub.0), at least a second dead time of a second dead time duration (Y) is introduced so that the effective pumping duration (TP) is equal to TP.sub.0 -Y; PA1 a third dead time having a third dead time duration (Z) is optionally introduced into each cycle; PA1 the feed duration (TA) is between 5 and 60 seconds, preferably between 10 and 40 seconds; PA1 the pumping duration (TP) is between 5 and 60 seconds, preferably between 10 and 40 seconds; PA1 the prefixed production pressure (PP) is between 10.sup.5 and 10.sup.7 Pa, preferably between 10.sup.5 and 10.sup.6 Pa; PA1 the gas stream to be separated comprises nitrogen and at least one less polar gaseous component, especially oxygen and/or hydrogen, and preferably the gas stream is air, the first gaseous component being nitrogen and the second gaseous component being oxygen, the air being, within the context of the present invention, the air contained inside a building or a heated or unheated enclosure, or the outside air, that is to say under atmospheric conditions, taken as it is or optionally pretreated; PA1 the first gaseous component is nitrogen and the second gaseous component is oxygen, and an oxygen-rich gas stream, that is to say comprising in general at least 90% oxygen, is produced; PA1 the high adsorption pressure is between 10.sup.5 Pa and 10.sup.7 Pa, preferably about 10.sup.5 Pa to 10.sup.6 Pa, and/or the low desorption pressure is between 10.sup.4 Pa and 10.sup.6 Pa, preferably about 10.sup.4 Pa to 10.sup.5 Pa; PA1 the feed temperature is between 10.degree. C. and 80.degree. C., preferably between 25.degree. C. and 60.degree. C.
However, it is known that the efficiency of separation of a gas mixture, such as air, depends on many parameters, especially the high pressure, the low pressure, the type of adsorbent material used and the affinity of the latter for the components to be separated, the composition of the gas mixture to be separated, the adsorption temperature of the mixture to be separated, the size of the adsorbent particles, the composition of these particles and the temperature gradient established inside the bed of adsorbent.
Currently, zeolites are the adsorbents most used in PSA processes. The zeolite particles usually contain monovalent, divalent and/or trivalent metal cations, for example cations of alkali metals, alkaline-earth metals, transition metals and/or lanthanides, these cations being incorporated during the synthesis of the zeolite particles and/or inserted subsequently using an ion-exchange technique, that is to say, in general, by bringing the unexchanged or raw zeolite particles into contact with a solution of one or more metal salts comprising the cation or cations to be incorporated into the zeolitic structure and subsequently recovering the particles of exchanged zeolite, that is to say of zeolite containing a given amount of metal cations. By way of example, mention may be made of zeolites of type X or LSX (Low Silica X) containing more than 80%, or even more than 90%, of metal cations such as, especially, lithium, calcium and/or zinc cations.
Such zeolites are especially described in documents EP-A-486,384, EP-A-606,848, EP-A-589,391, EP-A-589,406, EP-A-548,755, EP-A-109,063 and EP-A-760,248.
However, a recurrent concern in gas production using the PSA process, and in particular the VSA process, is to be able to offer the customer or the site using the gas produced, such as a combustion furnace for example, various combinations of production output and production pressure depending on the requirements and/or needs specific to this customer or this user site, and to do so by means of a standard PSA, particularly VSA, apparatus or unit installed on the site.
In other words, to be able, beyond a chosen nominal operating point of the PSA unit, to optimally adapt to the fluctuations in demand by the customer or the user site, so as to guarantee acceptable performance and an acceptable production cost from an industrial standpoint, despite the fluctuations in demand.
To do this, it is common practice to control the PSA or VSA unit so as to be able to obtain the combinations of production output and production pressure that are desired by the customer.
There is considerable teaching in the prior art concerning the control of PSA, particularly VSA, processes which make it possible to respond, at a given industrial unit, to changes in production gas consumption without inordinately affecting the overall energy consumption of the process.
Thus, a first known type of control is based on the introduction of a dead time, of constant or variable duration, into the production cycle, as described especially in documents EP-A-458,350 and EP-A-819,463.
During this dead time, the adsorbers are isolated, the machines operating with no load, and the intended objective is therefore to improve the energy consumption which, in the absence of this dead time, would be degraded in direct proportion to the reduction in the output consumed.
However, it turns out that this type of control allows specific energy degradation during reduced operation of the PSA, particularly VSA, unit to be limited only very partially. Thus, for a production of 50% of the nominal output of a VSA unit, the specific energy is degraded by more than 30% compared with the nominal value.
Furthermore, for batch production processes, especially in the case of a VSA unit having one or two adsorbers, there is necessarily a reduction in the pressure of the production gas output by the VSA, which must be compensated for by additional compression downstream of the unit.
In other words, the pressure variation DP in the production capacity is, for batch production units, for example of the MPSA type, such that: ##EQU1##
where:
In other words, in most cases, this type of control with introduction of dead time may not be sufficient by itself to maintain a perfectly constant service, especially for a fixed purity and a fixed pressure of the oxygen produced and whatever the output consumed, and therefore assumes that a second control be carried out downstream of the PSA or VSA unit, for example by means of a compressor.
An alternative solution involves decreasing the feed rate of the PSA or VSA unit, that is to say reducing the amount of material injected into the system during the adsorption phase of the sieve bed.
This may be achieved either by modifying the feed duration, as described by documents FR 97/16086 or U.S. Pat. No. 4,539,019, or by adjusting the aperture of an adsorber recompression valve, as recommended by document U.S. Pat. No. 5,258,056.
Although this solution is quite often preferable to the previous one in terms of energy consumption, given that it makes it possible, in certain cases, to maintain a constant specific energy up to 85% of the nominal output, as described by FR 97/16086, it does have in particular the disadvantage of leading to an obligatory reduction in the pressure of the production gas since the high pressure of the pressure cycle is necessarily lowered.
In addition, as in the case of the first control method, this second solution may not be sufficient to maintain a constant service. This is because it has been found that reduced operation, for example at 85% of the nominal output, leads to a drop in production pressure of 12,000 Pa.
Other documents recommend, conversely, specifically controlling the low pressure by modifying the gas streams during the cycle. In this regard, mention may be made of document EP-A-0,689,862.
However, the latter solution is quite complex to implement and often requires additional control means, compared with the previous variants, and, moreover, for units with positive-displacement machines, results in a modification to the high pressure of the cycle and therefore to the production pressure.
Furthermore, it seems that no system or process for controlling a PSA or VSA unit allowing the pressure of the gas delivered to be marginally adjusted, either in order to follow an occasional fluctuation in the needs of the customer or the consumer site, or in order to meet various minimum production pressure requirements, for example for various applications of the gas thus produced, with a single range of standard units, has hitherto been described.